MAINT-5232: Clean up some dubious LLSingleton methods.

Remove evil getIfExists() method, used by no one.
Remove evil destroyed() method, used in exactly three places -- one of which
is a test. Replace with equally evil instanceExists() method, which is used
EVERYWHERE -- sigh.

indra/llcommon/llregistry.h

@@ -269,7 +269,7 @@ public:
 
 		~ScopedRegistrar()
 		{
-			if (!singleton_t::destroyed())
+			if (singleton_t::instanceExists())
 			{
 				popScope();
 			}

indra/llcommon/llsingleton.h

@@ -166,31 +166,19 @@ public:
 		return NULL;
 	}
 
-	static DERIVED_TYPE* getIfExists()
-	{
-		return sData.mInstance;
-	}
-
 	// Reference version of getInstance()
 	// Preferred over getInstance() as it disallows checking for NULL
 	static DERIVED_TYPE& instance()
 	{
 		return *getInstance();
 	}
-	
-	// Has this singleton been created uet?
-	// Use this to avoid accessing singletons before the can safely be constructed
+
+	// Has this singleton been created yet?
+	// Use this to avoid accessing singletons before they can safely be constructed.
 	static bool instanceExists()
 	{
 		return sData.mInitState == INITIALIZED;
 	}
-	
-	// Has this singleton already been deleted?
-	// Use this to avoid accessing singletons from a static object's destructor
-	static bool destroyed()
-	{
-		return sData.mInitState == DELETED;
-	}
 
 private:
 

indra/llcommon/tests/llsingleton_test.cpp

@@ -65,7 +65,6 @@ namespace tut
 
 		//Delete the instance
 		LLSingletonTest::deleteSingleton();
-		ensure(LLSingletonTest::destroyed());
 		ensure(!LLSingletonTest::instanceExists());
 
 		//Construct it again.

indra/newview/llfloaterimcontainer.cpp

@@ -101,7 +101,7 @@ LLFloaterIMContainer::~LLFloaterIMContainer()
 	gSavedPerAccountSettings.setBOOL("ConversationsMessagePaneCollapsed", mMessagesPane->isCollapsed());
 	gSavedPerAccountSettings.setBOOL("ConversationsParticipantListCollapsed", !isParticipantListExpanded());
 
-	if (!LLSingleton<LLIMMgr>::destroyed())
+	if (LLIMMgr::instanceExists())
 	{
 		LLIMMgr::getInstance()->removeSessionObserver(this);
 	}